For a non-VBA solution..

If your sentences are in column A, then in B1 use the following formula and fill down:

=IF(OR(LEFT(LOWER(A1),FIND(" ",A1)-1)={"fox","horse","cow"}),MID(A1,FIND(" ",A1)+1,LEN(A1)),A1)

Copy the results from column B over the original values in column A (Copy, then PasteSpecial > Values).